Epi's Pork Pot Roast (Pernil al Caldero al Estilo de Epi)

1 (4-pound) pork shoulder or butt
1 tablespoon salt
1 tablespoon black pepper
2 tablespoons dried oregano
4 large garlic cloves, peeled
1/3 cup corn oil
2 to 3 cups water or chicken stock

Remove any excess fat and skin from the meat. Make small cuts all over. Wash dry. In a mortar, combine the salt, pepper, oregano, garlic, and oil. Crush all of the ingredients to make a paste. Rub the meat with this mixture. Place it in a bowl and cover it with plastic wrap. Refrigerate for 2 hours. In a large pot, brown the meat over high heat. Lower the heat and add the water or stock. Cook, covered, over medium low heat for 3 hours. Add more liquid if necessary. The pot roast is done when the meat shreds easily. To serve, cut the meat off the bone.